The Blind
Spot

One of the most dramatic experiments to perform is the demonstration of
the blind spot. The blind spot is the area on the retina without
receptors that respond to light. Therefore an image that falls on this
region will NOT be seen. It is in this region that the optic nerve exits
the eye on its way to the brain. To find your blind spot,
look at the image below or draw it on a piece of paper:

To draw the blind spot tester on a piece of paper, make a small dot on
the left side separated by about 6-8 inches from a small + on the right
side. Close your right eye. Hold the image (or place your head from the
computer monitor) about 20 inches away. With your left eye, look at the
+. Slowly bring the image (or move your head) closer while looking at the
+. At a certain distance, the dot will disappear from sight...this is
when the dot falls on the blind spot of your retina. Reverse the process.
Close your left eye and look at the dot with your right eye. Move the
image slowly closer to you and the + should disappear.

Here are some more images that will help you find your blind spot.

For this image, close your
right eye. With your left eye, look at the red
circle. Slowly move your head closer to the image. At a certain
distance, the blue line will not look broken!! This is because your
brain is "filling in" the missing information.

This next image allows you to see another way your brain fills in the
blind spot. Again, close your right eye. With your left eye, look at the
+. Slowly move your head closer to the image. The space in the
middle of the vertical lines will disappear.

In the next two images, again close your right eye. With your left eye,
look at the numbers on the right side, starting with the number "1." You
should be able to see the "sad face" (top image) or the gap in the blue
line (bottom image) in your peripheral vision. Keep your
head still, and with your left eye, look at the other numbers. The sad
face should disappear when you get to "4" and reappear at about "7."
Similarly the blue line will appear complete between "4" and "7."

Here is another image to show your blind spot. Close your right eye.
With your left eye, look at the +. You should see the red dot in your
peripheral vision. Keep looking at the + with your left eye. The red
dot will move from the left to the right and disappear and reappear as
the dot moves into and out of your blind spot.

An octopus does not have a blind spot!
The
retina of the octopus is constructed more logically than the mammalian retina. The photoreceptors in the octopus
retina are located in the inner portion of the eye and the cells that
carry information to the brain are located in the outer portion of the
retina. Therefore, the octopus optic nerve does not interrupt any space
of retina.

Depth
Perception - I

For grades K-12

Two eyes are better than one, especially
when it comes to depth perception. Depth perception is the ability to
judge objects that are nearer or
farther than others. To demonstrate the difference of using one vs. two
eye to judge depth hold the ends a pencil, one in each hand. Hold them
either vertically or horizontally facing each other at arms-length from
your body. With one eye closed, try to touch the end of the pencils
together. Now try with two eyes: it should be much easier. This is
because each eye looks at the image from a different angle. This
experiment can also be done with your fingers, but pencils make the effect
a bit more dramatic.

Materials:

Pencils (but your fingers make a good substitute)

Drop IT! - Depth Perception - II

For Grades 3-12

Here's another demonstration of the importance of two eyes in judging
depth.
Collect a set of pennies (or buttons or paper clips). Sit at a table with
your subject. Put a cup in front of your subject. The cup should be
about two feet away from the subject. Have your subject CLOSE one eye.
Hold
a penny in the air about 1.5 ft. above the table. Move the penny around
slowly. Ask your subject to say "Drop it!" when he or she thinks the
penny will drop into the cup if you released it. When the subject says
"Drop it," drop the penny and see if it makes it into the cup. Try it
again when the subject uses both eyes. Try it again with the cup farther
away from the subject. Try it again with the cup closer to the subject.
Compare the results of "10 drops" at each distance.

Questions:

Is there improvement with two eyes?

Is there improvement with the cup is closer to the subject?

Materials:

Cup (yogurt cup or drinking cup)

Drop objects (pennies, buttons, paper clips, clothes pins)

OR TRY THIS GAME

Get a piece of paper and draw a target
similar to the one on the right. The actual dimensions of the circles are
not too important and you don't have to color the circles. Place the
target on the ground about five feet in front of you. Have a friend stand
near the target. Have your friend hold out an ink marker with the tip
pointing down. Close one eye. Tell your friend to move forward or
backward or side to side until you think the marker would hit the center
of the target if it was dropped. Tell your friend to drop the marker when
you think the marker is over the target center. The marker should leave a
spot where it hit the target. Try it 10 times with one eye closed and add
up the "score" for the 10 drops. Now try it with both eyes opened (get a
different color marker when you use 2 eyes to see the difference on the
target). Is your score better when you use two eyes?

Materials:

Paper for target

Markers (two colors)

Shifting Backgrounds, Shifting Images

For grades K-12

Here's another way to demonstrate how different images are projected on to
each eye. Look at an object in the distance (20-30 feet away), such as a
clock on the wall. Close one eye, hold up your arm and line up your
finger with the object. Now without moving your finger or your head,
close the opened eye and open the closed eye. The object in the distance
will appear to jump to the side...your finger will no longer be lined up.
This shows that different images fall on each eye.

Materials:

NONE

Dark Adaptation

For grades 3-12

There are two types of photoreceptors in the eye: rods and cones. The
rods are responsible for vision in dim light conditions, the cones are for
color vision. To demonstrate how the photoreceptors "adapt" to low light
conditions, get a collection of objects that look slightly different: for
example get 10 coke bottle caps, 10 soda bottle caps, and 10 water bottle
caps. They should feel the same, but not look the same. In a bright
room, ask students to separate the caps into piles of similar caps. Then
turn off the lights so the room is very, very dim. Ask them to separate
the caps again. Turn off the lights and look at the results...there
should be many mistakes. Count the number of errors. Then dim the lights
again and talk/discuss about dark adaptation or about the animals that can
see in the dark. The technical explanation for dark adaptation is not
necessary for small children. Plan to talk and discuss for about 7-10
minutes...this will be enough time for a least partial adaptation of the
photoreceptors. After the discussion (7-10 minutes), ask the students to
separate the caps again in the same very, very dim conditions as before.
Count the number of errors. There should be fewer errors this time
because the photoreceptors have adapted to the low light conditions.

Visual Illusions

What you see is not always what is there. Or is it? The eye can play
tricks on the brain. Here are several illusions that demonstrate this
point.

The Magic Cube

Look at the center cube. What side is the front? Is the front as shown
on the cube on the right side or is the front as shown on the cube on the
left side or is there no front at all?

Which of the lines shown below is longer?

Muller-Lyer Illusion

Measure them. You may be surprised to find out that they are the same
length. We see the lines as different because we have been "taught" to
use specific shapes and angles to tell us about size.

More examples of the Muller-Lyer Illusion

Stare at the middle of picture with black squares 15-30 seconds. Are
those really dots that appear at the corners of the squares? What happens
if you focus on a dot? Now look at the middle of the picture with the
white squares. Do you see dots again? What color are they?

Here is another example of the same illusion.

Are the locations of the spots different in these two pictures? Why?

Do you see a vase or a face in the figure below? This type of picture
was first illustrated by psychologist Edgar Rubin in 1915. Notice that it
is very difficult to see both the faces and the vase at the same time.
This may happen because we tend to focus our attention on only one part of
the image...either the faces or the vase.

Stare at the yellow + in the middle of figure for 15-30 seconds. Then
move your gaze off to the white square on the right. Did the colors
really reverse themselves? This is an example of an "afterimage".

Here's another example of creating an afterimage. Can you put the
fish in the bowl? Try this. Stare at the yellow stripe in the middle of
the fish in the picture below for about 15-30 sec. Then move your gaze to
the fish bowl. You should see a fish of a different color in the bowl.
It helps if you keep your head still and blink once or twice after you
move your eyes to the bowl. The afterimage will last about five
seconds.

Try these two interactive demonstrations (using JAVA-capable browsers) of
afterimages:

What's Happening: in the retina of your eyes, there are three types of color
receptors (cones) that are most sensitive to either red, blue or green.
When you stare at a particular color for too long, these receptors get
"tired" or "fatigued." When you then look a different background, the
receptors that are tired do not work as well. Therefore, the information
from all of the different color receptors is not in balance. Therefore,
you see the color "afterimages."

Stare at the + for about 15 seconds, then
shift your gaze to the
right side of the image.

Stare at the + for about 15 seconds, then
shift your gaze to the
right side of the image.

Do the lines on the right side of the image
look straight? Are they really straight?

7. The Poggendorff Illusion was created by Johann Poggendorff in 1860.
Are the lines behind the rectangles straight or not? It looks as if it
does not go straight across, but does it?

Poggendorff
Illusion

8. Hmmm...is the center circle on the right the same size as the center
circle on the left? For many people it appears that the circle that is
surrounded by the small circles is larger that the circle that is
surrounded by the larger circles. However, I know that they are the same
size....I copied and pasted the same exact circle into the middles!!

Titchener Illusion

This
illusion shows that our brains judge size by comparing objects to things
in the surroundings.

9. The Wundt
Illusion. Which arc is larger? You might see that the top one is
smaller, but they are the same size. The top one looks smaller because
the shorter arc of the top figure is next to the large arc of the bottom
figure.

10. Is this book opening toward you or away from you?

11. To tee or not to tee...that is the question. This
inverted "T" has two lines....are they the same length? You bet they
are...I copied one line and pasted it on the bottom of the first line.
Measure them yourself.

12. Which arc comes from the circle with the largest
diameter?

It probably looks like arc C is part of the largest circle. However, all
the arcs are actually from the SAME circle. Look at the same figure again
- however, this time I have blocked the right and left sides of the larger
two arcs. Each arc comes from a circle of identical size.

13. Subjective Contours: Filling the gap. Your brain
tries to fill in these four pictures with images that really are not
there. Do you see
a:

Cube?

Triangle?

Square?

Rectangle?

14. Baldwin Effect: The distance between the two large boxes
is the same as the distance between the two small boxes. For many
people, the distance between the small boxes appears larger.

For pages of more illusions with their physiological
explanations, see:

Context Effect

Simultaneous Contrast

How does the surrounding color influence what we see? Find out with
this interactive picture. You must have a
browser that supports "JAVA scripts".

Don't Jump to Conclusions

For Grades 3-12

How does your brain prepare you to see something? Find out with
this
interactive picture. You must have a browser that supports "JAVA
scripts".

Cow Eye
Dissection

For Grades 9-12

The
Exploratorium in San Francisco has a worthwhile virtual Cow Eye
Dissection to check out.

See It to Believe It - Visual Discrimination

For Grades 3-12

How good are you at seeing different colors? Let's find out. Put
equal amounts of water into 5 to 10 different containers (paper cups,
drinking glasses, yogurt containers all work well). Then put one drop of
FOOD COLORING into one of the containers of water. Put two drops of food
coloring into the next container, 3 drops of food coloring into the next
container and so on. Label the cups with a secret code so you know how
many drops of food coloring went into each cup.

Questions and Comparisons

Have students
arrange the colors from lightest to darkest. Keep track of where mistakes
are made.

Try different colors of food coloring.

Start with more
or less water in the container to make it more or less difficult to tell
the colors apart.

Try different lighting conditions:

Dim vs
Bright Light

Outside (natural) vs Inside (artificial) Light

Fluorescent vs Incandescent Light

Materials:

Food coloring: red, blue, yellow (mix them to make more colors)

Water

Containers

Eyedropper

X-Ray Vision??

Grades 3-12

Do you have "X-Ray
Vision?" You may be able to see through your own hand with this simple
illusion. Roll up a piece of notebook paper into a tube. The diameter of
the tube should be about 0.5 inch. Hold up your left hand in front of
you. Hold the tube right next to the bottom of your left "pointer" finger
in between you thumb (see figure below).

Look through the tube with your RIGHT eye AND keep
your left eye open too. What you should see is a hole in your left hand!!
Why? Because your brain is getting two different images...one of the hole
in the paper and one of your left hand.

Materials:

Notebook paper

Star Light, Star Bright

Grades 3-12

Have you ever
noticed that it is easy to see a star in the sky by NOT
looking directly at it? It is actually easier to see a dim star at night
by looking a bit off to the side of it. Try it! This is because the two
types of photoreceptors (rods and cones) in the retina perform different
functions and are located in the retina in different locations. The
cones, which are best for detail and color vision, are in highest
concentration in the center of the retina. The rods, which work better in
dim light, are in highest concentration in the sides of the retina. So if
you look "off-center" at the star, its image will fall on an area of
the retina that has more rods!

Materials:

None

Color Cards

Grades K-3

Here is a fun way to introduce and explore the sense of vision. Get a
variety of sample "color cards" from your local paint store. These cards
are about the size of index cards and show the variety of paint that is
available. Bring them back to class and have students match up similar
colors. You can also use samples of gift wrap or wall paper to make color
or pattern cards. Just glue the wrap or wall paper to a piece of card
board to get yourself a "Color Card."

Materials:

Sample color cards from a paint store

Wall paper samples

Gift wrap

Scissors, glue and cardboard (if you will make your own cards)

Color Spy

Grades K-3

Color Spy is a variation of the "I Spy" game. Divide players into teams.
Write the words "blue", "red", "yellow", "orange" and "green" on separate
pieces of paper. Have one member of each team pick a paper.
The color picked will be the name of the team.

When someone says "Go," the teams will have 10 minutes to look around the
room for objects that have their team's color. Teams must make a list of
all the objects they find. After the 10 minute search period, the teams
come back together and the lists of objects are read. Each team gets one
point for each object found. After the lists are read, each team will get
five minutes to search the room for colored objects that the
other teams did NOT find. For example, if the red team did
not find a red apple, another team that DID find the red apple will get
one point. The team with the most total points after both searches is the
winner.

Materials:

Pencils and paper

Seeing in the Dark

Grades K-12

Of course you cannot see if it is completely dark, but you can see a bit
in dim light. In dim light, the receptors in
your eyes called rods are doing most of the work. However, the rods do
not provide any information about color. The other photoreceptors in your
eye, called cones, are the ones that are used for seeing color. The cones
do not work in dim light. That's why you cannot see colors in dim light.
Check it out for yourself:

Get five pieces of paper of different colors (such as different colored
typing paper or construction paper). Dim the lights until you can just
barely see. Wait about 10 minutes (maybe listen to some music while you
wait). Then write on each piece of paper the color you think that paper
is. Turn on the lights and see if your guesses were correct. Did everyone
in your class mix up the same color or did everyone get the colors
correct?

Materials:

Pencils or pens

Colored paper (about five different colors)

Accommodating Accommodation

Grades 3-12

When light enters the eye, it is first bent (refracted) by the cornea.
Light is bent further by the lens of the eye in a process called
accommodation. To bring an image into sharp focus on the retina, the
lens of the eye changes shape by bulging out or flattening. A flatter
lens refracts less light. Here's how to demonstrate accommodation:

Close one eye and stare at a point about 20 feet away. It should be in
focus. Keep focusing on the point and raise one of your fingers into your
line of sight just below the point. Your finger should be a bit blurred.
Now, change focus: look at the tip of your finger instead of the point 20
feet away. Your finger will come in focus, but the distant point will be
blurred.